How do they figure out the number of calories in a recipe?

Chef Richard Rosendale opens a cabinet of spices he uses to experiment with recipes in a bunker below the Greenbrier Resort in White Sulphur Springs, West Virginia.

BRENDAN SMIALOWSKI/AFP/Getty Images

Recipe developers, cookbook companies and the like use databases to help determine the calorie counts in their dishes, says Jessica Cox, a registered dietician and recipe developer who has consulted on cookbooks. But since cooking a food can affect both the number of available portions and their caloric content, you have to make sure you know whether to look at a food's raw calorie count or cooked calorie count.

When you're preparing a fatty, 3-pound (1.4-kilogram) pork roast, for example, some of the fat will cook off, reducing the weight of the roast. So if Cox had to analyze your pork roast recipe, she would first find the calories for a 3-pound (1.4-kilogram) raw roast, using the USDA's National Nutrient Database for Standard Reference. Then she would prepare the roast to determine its yield, or final weight, which in turn determines the number of portions available. The standard serving size for meat is 2-3 ounces (57-85 grams) [source: American Heart Association]. Let's say your roast cooks down to 2.5 pounds (1.1 kilograms). That would mean you now have fewer portions available to eat — about 13 servings in the cooked version versus 16 servings in the raw roast.

Interestingly, each 3-ounce serving of your cooked roast will actually have more calories than a 3-ounce serving of its raw cousin. That's because the raw meat contains both water and fat, says Cox, and more water is lost during the cooking process than fat. This means the resulting portions will contain a higher concentration of fat, and thus calories. Here's the comparison using the USDA's National Nutrient Database for Standard Reference:

Cox says recipe developers and testers also make sure they're only looking at the calories in the portions of a food available for consumption. So they'll only count the calories in the trimmed asparagus, for example, not those in the woody portions tossed away before cooking. They will even determine the percentage of a marinade that's left in the bowl after the meat is removed, subtracting out those calories from the dish's total count.

One caveat for consumers: Not all cookbook publishers or recipe websites hire experts like Cox to ensure their recipes' nutritional information is as accurate as possible. "It is impossible to know how different publications are calculating their calories," she says, adding the subject of calorie counts "is a kind of gray thing."